Pressure is a physical quantity that corresponds to the ratio of force acting on a surface to the area of the surface. A device that can be used as a gauge to measure the pressure is a pressure sensor.
Atmospheric pressure is the pressure exerted at the surface by a column of air in an atmosphere. The atmospheric pressure varies with altitude and weather patterns. The quantity obtained with a pressure sensor may exclude the ambient atmospheric pressure and in this case indicates overpressure or relative pressure. If atmospheric pressure is included, the result indicates absolute pressure. The pressure sensor may also be arranged to measure the pressure difference between two ambients in which case the measured quantity is called differential pressure.
Micro-Electro-Mechanical Systems, or MEMS can be defined as miniaturized mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. Since MEMS devices are created with the same tools used to create integrated circuits, micromachines and microelectronic elements can be fabricated on a piece of silicon to enable various types of devices.
FIG. 1 illustrates an exemplary structure of a microelectromechanical device for sensing of pressure, shown in page 204 of “Practical MEMS” by Ville Kaajakari. Microelectromechanical pressure sensors typically comprise a thin diaphragm 10 that is spanned over a gap in an insulator layer 12. The gap forms a volume for a reference pressure, and the diaphragm deforms due to a difference between the reference pressure and an ambient pressure surrounding the sensor. The diaphragm displacement may be translated to an electrical signal with capacitive or piezoresistive sensing.
Conventionally, the diaphragm 10 is formed of a thinned area made by etching into a silicon wafer, as shown in FIG. 1. The diaphragm 10 is a very thin foil, so the etched recess is almost as deep as the whole silicon wafer. Lately, the desire in the field has been to produce remarkably thinner pressure sensor structures. It has been noted, however, that reduction of thickness of the structure imposes many challenges to the design. In a microelectromechanical pressure sensor, the detected diaphragm displacement due to pressure change can be nanometers or less, and also signals generated by the diaphragm displacements are very small. Already change of dimensions of the sensor structure elements caused by thermal expansion in varying operating temperatures can cause significant variations to the generated measurement signals. In addition, the sensor structure and the associated electronics need to be capsulated in a package. The package may have different thermal expansion coefficient than the sensor structure, which may cause large, temperature dependent bending stresses that distort the measurements.